1. Field of the Invention
This invention relates to a driving force distribution method and a driving force distribution apparatus for four-wheel-drive vehicle that has a torque transmission device.
2. Discussion of the Background
At a four-wheel-drive vehicle of recent years, front wheels are always driven, and rear wheels are selectively driven depending on its necessity by arranging a torque transmission device on the way of a propeller shaft that connects an engine and the rear wheels. Such torque transmission device is controlled so as to transmit torque to the rear wheels, which is calculated based on rotational speed differences between the front and the rear wheels, for instance. As for a fail-safe of the torque transmission device, if some abnormality happens, the torque transmission device is controlled not to transmit any torque, i.e. the vehicle becomes a two-wheel-drive. However, it is possible that the vehicle sticks if the abnormality happens when the vehicle is on a rough road such as snow-covered road, because of two-wheel-drive.